1. Field of the Invention
The present invention relates to power monitoring and control apparatuses for display devices, and in particular, to a power monitoring and control apparatus for a display device that protects a power supply circuit element by detecting a malfunction in power supplied to the display device when the display device and an information processing device are connected via a DVI interface.
2. Description of the Related Art
Today, a digital DVI (Digital Visual Interface) is employed in addition to an analog RGB interface as an interface for connecting a display device such as an LCD (hereinafter also referred to as a monitor) and an information processing device such as a personal computer (hereinafter also referred to as PC).
In the DVI, mainly, three standardized signals (a TMDS signal and a clock, a DDC signal and a clock, and a hot plug signal) are transmitted. Among these signals, the TMDS signal and a clock thereof with which video signals are transmitted at a high speed are generated by a supply voltage (+3.3 V) on a side of the PC, and an impedance thereof matches an impedance of the supply voltage on a side of the monitor.
FIG. 6 shows a circuit block diagram of a portion relating to power monitoring of a conventional display device.
In the drawing, an essential configuration of a portion relating to a DVI 30 between a monitor 10 and a PC 20 is illustrated. The monitor 10 mainly includes a power plug 4 connected to a commercial power source (AC 100 V) 40, an AC/DC conversion unit 1 that converts an alternating voltage 100 V inputted from the power plug 4 into a direct voltage, a REG 2 that supplies +3.3 V to a receiver 3, and the receiver 3 that receives a group of TMDS signals.
The DVI 30 which is an interface between the monitor 10 and the PC 20 mainly includes a group for transmitting the TMDS signal, a group for transmitting the DDC signal, and a hotplug (HPG) signal.
The group of the TMDS (Transition Minimized Differential Signaling) includes a video signal and a clock signal, and is transmitted from a signal transmission unit 21 of the PC 20 to the receiver 3.
The group of the DDC (Display Data Channel) includes a DDC data signal and a direct current signal of +5 V (DDC +5 V), and is transmitted from a DDC transmission unit 22 of the PC 20 to the monitor 10.
The hotplug (HPG) signal is a signal that enables plugging in and out of an interface cable while the power of the monitor is on, and is transmitted from the monitor 10 to a hotplug detection unit 23 of the PC 20 along with DDC +5 V.
The TMDS signal transmitted from the signal transmission unit 21 is generated by the supply voltage (+3.3 V) on the side of the PC 20, and its impedance matches the impedance of the supply voltage of +3.3 V supplied to the receiver 3 on the side of the monitor 10.
Further, the monitor 10 is provided with a circuit block for generating the hotplug (HPG) signal from the DDC +5 V signal. This circuit block is includes transistors (T1 and T2) and a group of resistances (R1 to R5).
When DDC +5 V is supplied from the PC 20 to the side of the monitor 10 to supply the supply voltage +3.3 V on the side of the monitor 10, the hotplug HPG signal is in a HIGH state at +5 V via the resistance R3. The hotplug detection unit 23 determines that the monitor is in a power supplied state when detecting that the HPG signal is HIGH.
On the other hand, if the power plug 4 of the monitor 10 is unplugged while both of the monitor 10 and the PC 20 are in the power supplied state, the voltages of +5 V and +3.3 V supplied to the REG 2 and the receiver 3 of the monitor stop and decrease to 0 V. In this case, since the voltage of +3.3 V is still being applied to the signal transmission unit 21 on the side of the PC 20, a potential difference is generated between the signal transmission unit 21 on the side of the PC and the receiver 3 on the side of the monitor.
Therefore, an abnormal current over the rated current flows from the signal transmission unit 21 of the PC 20 via the DVI 30 to the receiver 3 of the monitor 10. Because of this abnormal current, there is a case in which a circuit element that constitutes such as the receiver 3 or the signal transmission unit 21 is destroyed (see Japanese Unexamined Patent Publication No. 2003-209920, for example).
According to power monitoring of the conventional display device, in order to reduce the flow of the abnormal current over the rated current, the impedance of the signal transmission unit 21 is controlled using the hotplug HPG signal. An interface control unit 24 of the PC 20 is a portion for controlling the impedance of the signal transmission unit 21 in response to the hotplug HPG signal.
When the supply voltage of +3.3 V on the side of the monitor 10 is being supplied, the hotplug detection unit 23 detects that the HPG signal is in the HIGH state, the interface control unit 24 allows the signal transmission unit 21 to transmit a display signal.
On the other hand, when the power plug 4 of the monitor 10 is unplugged and the voltage of +3.3 V on the side of the monitor is not supplied, the transistor T2 is turned OFF and the HPG signal becomes LOW.
When the hotplug detection unit 23 detects that the HPG signal becomes LOW, the interface control unit 24 controls so that the DVI 30 connected to the signal transmission unit 21 becomes a high impedance.
By control such that the abnormal current does not flow from the PC 20 to the monitor 10, it is intended to protect devices of the receiver.
However, when the TMDS signal becomes high impedance merely based on change of the state of the HPG signal from HIGH to LOW, there still remains the potential difference between the signal transmission unit 21 and the receiver 3, and setting of a detection voltage of the receiver 3 is difficult due to such as a hysteresis and a threshold value of a base voltage of the transistor T2.
Further, in order to avoid such a difficulty in setting, it is generally designed so that the supply of the power of +3.3 V on the side of the monitor 10 is kept as it is even when a power button is pressed to perform a power-off operation. In other words, because the power supply of +3.3 V on the side of the monitor 10 is maintained even when a user performs the power-off operation, power is consumed even while unused, and therefore it is not compliant with a trend of energy conservation.
On the other hand, the most effective way to meet the recent demand for energy conservation is to stop the power supply of +3.3 V on the side of the monitor when performing a power-off operation. Further, in order to protect the devices such as the receiver, it is required to reduce the flow to the monitor of the abnormal current as described above immediately after the supply of +3.3 V is stopped.